The present invention generally relates to a multiple-feature telephone unit wherein the various features are provided by circuitry integrated within a single integrated circuit. The present invention more particularly relates to such an integrated circuit which includes an audio processor, a keypad monitor, a tone ringer control, and a liquid crystal contrast control adapted for use within an Integrated Services Digital Network (ISDN) system and controllable by the microprocessor of the system over a serial bus generally used in such systems.
Multiple-feature telephones are well known in the art. Such telephones are known to include such functions as hands-free or speakerphone operation, multiple-line control, hold, intercom, selectable tone ring, and liquid crystal display. To provide such functions, a multiple-feature telephone unit requires an audio processor adapted to be coupled to a speaker and a microphone, a keypad monitor for enabling the activation of the various features selected on a keypad, a tone ringer and control for producing any one of a plurality of ringing tones, and a contrast control circuit for adjusting the contrast of the liquid crystal display.
Multiple-feature telephones incorporating some or all of the above features are also known for use in digital telephone network systems such as Integrated Services Digital Network (ISDN) systems. However, such known multiple-feature telephone units are relatively expensive because they include many discrete components. Also, these prior art telephone units are not readily adapted for being added to an existing ISDN system not having such features.
The present invention overcomes these deficiencies by providing a single integrated circuit which includes all of the circuitry necessary for a multiple-feature telephone unit. Because all of the circuitry is incorporated onto a single integrated circuit, a telephone unit embodying the present invention is less expensive than prior units. Further, the integrated circuit includes a digital interface for supporting a serial bus typically used in ISDN systems and, hence, is fully compatible therewith.